Sky Clock Information Reference

Understanding the Clear Sky Clock

One of the several tools we have on the LVAS website is the Clear Sky Clock. This color coded report is a prediction tool that forecasts an area’s Cloud Cover, Transparency, Seeing, Darkness, Wind effect, Humidity, and Temperature for the next 48 hours. Based on weather and atmosphere data, the Canadian Meteorological Center (CMC) compiles, models and reports sky observation forecasts for Canada and the United States. It is one of the few meteorological sites that offer tailored weather reporting for astronomers. However, interpreting the complicated CMC data can be quite a task for a layperson. Enter Mr. Attilla Danko, the author of the Clear Sky Clocks. Mr. Danko developed several programs that utilize the CMC data and display it in location specific color-coded tables. Using these tables an astronomer can get a fairly comprehensive prediction of how productive a location will be for telescope observations. The Sky Clock contains two sections, the Sky (Cloud Cover, Transparency, Seeing, and Darkness) and the Ground (Wind, Humidity, and Temperature.) Each hour is represented by a colored block. In the example below the table starts on February, 13th, 2006 at 10pm on Monday. Each day’s date information is expressed in a different color and separated by a red vertical line to aid reading (Monday is green, Tuesday is yellow, and Wednesday is Red). I will quickly walk though each of the sections explaining how to read the color codes.

Sky – Optical conditions that an astronomer is predicted to experience when observing.

Cloud Cover: In this section, there is a forecast for Cloud Cover. The color code for cloud cover should be interpreted as; darker the color, the less overall cloud cover is predicted.

Transparency: The Transparency section forecasts how “clear” the atmosphere is predicted to be. This forecast is based on the average predicted amount of water vapor that will be in the air. The higher the transparency, the better it is for viewing low contrast objects such as nebulae and galaxies. Like the Cloud Cover color code, the darker the color the better the conditions. High contrast objects such as planets and globular clusters can be seen in poorer transparency conditions. If transparency data is not available (perhaps due to heavy cloud cover) the color code of white is used. The forecast does not take into account other factors that can affect Transparency such as pollutants, dust, smoke and other aerosols. 

Seeing: The next section predicts the turbulence in the air or “Seeing” conditions that might be experienced. The better the predicted Seeing, the easier it will be to view fine detail at high magnifications.  Poor or Bad Seeing conditions is typically caused by turbulence due to thermal layering (temperature differences at different atmospheric heights). The effect is that of blurring or rippling and results in poor details at high magnification. Like the preceding sections, the darker the color the better the Seeing conditions. Once again, the color white is used if there is no data available typically caused by heavy cloud cover. 

Darkness: While not a true weather forecast, the Darkness section shows when the sky is expected to be dark. Like most of the color codes, the darker the color the darker the conditions. Factors that affect this forecast are predicted moonlight and sunset. It does not take into account light pollution, dust, or cloud conditions. The scale below uses the limiting star magnitude (the magnitude of the faintest star that seen looking straight up). 

Ground – Weather conditions the astronomer is predicted to physically experience while observing.

Wind: The Wind forecast predicts the Wind conditions at the observation site. The darker the color, the better the wind conditions. In Moderate or Strong Wind conditions, telescopes such as large dobsonians may be difficult to use. 

Humidity: The table also includes a Humidity prediction. The higher the humidity, the more likely the conditions for telescope dewing. In the Las Vegas area, it is rare that humidity impacts viewing conditions. Like most of the color codes, the darker the color the better the conditions. 

Temperature: The last section predicts the Temperature conditions. While not directly affecting telescope performance, considerations should be made when there are large temperature differences between the telescope and ambient air. These large differences may require cool-down (or heat-up) time for telescope mirrors to reach equilibrium to prevent tube currents that can affect Seeing. There is no “better” end of the color scale for this section.